This invention relates to circuitry and methods which may be used to provide a current to a load. More particularly this invention relates to circuitry that provides a constant current to a load until the voltage across the load reaches a certain value. When this value is reached, the current delivered to the load must be varied to maintain a constant load voltage.
This type of circuitry is referred to as a constant-current/constant-voltage system, and the charging circuitry of a lithium ion battery is a common use for such circuitry. Frequently, conventional systems use a programmable resistor to set the value of the constant charging current.
It would be desirable to provide circuitry that provides a constant current to a load until the voltage across the load reaches a pre-determined value and then maintains the voltage across the load at the predetermined value by varying the current to the load.
It would also be desirable to provide a signal proportional to the load current.
It is an object of the invention to provide circuitry that provides a constant current to a load until the voltage across the load reaches a pre-determined value and then maintains the voltage across the load at the predetermined value by varying the current to the load.
It is also an object of this invention to provide a signal proportional to the load current.
The circuit according to the invention includes a first current loop that is adapted to provide a fixed current, a second current loop that is adapted to provide a variable current, and a priority circuit.
The priority circuit receives a first signal from the first current loop and a second signal from a second current loop. The first signal indicates the level of an available fixed current. The second signal from the second current loop indicates the level of an available variable current. The priority circuit may compare the two signals and select one of the first current loop and the second current loop to provide current to the load based on a predetermined priority assigned to the first signal and the second signalxe2x80x94e.g., whichever current has lower magnitude.
A method of charging a load according to the invention includes selecting a load charging current from one of a fixed current and a variable current, the variable current being based on the voltage across the load, the selecting being based on a predetermined priority.